An important surgical procedure, particularly in the case of trauma, is the restoration of peripheral nerve function. When a peripheral nerve is severed, the distal stump is unconnected to the central nervous system and cannot function. The severing of important peripheral nerves may seriously impair the function of appendages, and unless connection of the severed nerve is restored, the appendage is permanently disabled. For example, severing the radial, median or ulnar nerve of the arm will destroy to varying degrees the function of the arm and hand.
A nerve consists of a bundle of numerous fibers or axons through which the electrical signals that comprise nervous system impulses are transmitted. Surrounding the bundle of nerve fibers are Schwann cells which, in turn are surrounded by basal lamina, and additional connective tissue surrounds the whole nerve. In the case of a severed peripheral nerve in which the proximal and distal stumps can be approximated, rejoining of the nerve ends is generally accomplished by approximating the severed nerve ends and suturing their connective tissues together. In such cases, the regenerating axons can grow directly from the cut end of the proximal stump into the distal stump, and then on to their targets.
In many instances; however, the proximal and distal stumps may be separated by a distance which does not permit them to be directly approximated, necessitating the use of a shunt that promotes axon regeneration therethrough. One type of shunt may be a length of nerve obtained from another part of the body. For example, the sural nerve in humans is often cut to obtain a segment suitable for shunting a more important nerve elsewhere in a patient. The sural nerve is located in the leg and transmits sensations from the leg and foot, but does not transmit motor function impulses to the foot. Accordingly, it is felt desirable to use such a nerve to shunt more important motor function nerves, such as the radial, medial or ulnar nerves of the arm. Use of the sural nerve is desirably avoided because the loss of sensation occasioned by the severing thereof is of no mean consequence. Furthermore, removal of the sural nerve necessitates an operation of some consequence prior to the operation for restoring the severed nerve in which the removed sural nerve segment is to serve as a shunt.
It was discovered by Keynes et al. Brain Research 295 (1984), 275-281, that basal lamina of degenerating mammalian muscle provides surfaces which promote the regeneration of axons from the proximal stump of a severed nerve. It is believed that two substances, fibronectin and laminin, found in mammalian muscle basal lamina contribute to a conducive environment for axon regeneration. Thus, muscle specimens were obtained from mice and these tissue specimens were reimplanted into the mice, from which they were obtained, as a graft on the end of the proximal nerve stump, whereupon axon ingrowth into the tissue was observed. A problem with degenerating muscle tissue for use as a graft is its tendency to cause an intense inflammatory response in the host animal, including a human. This is particularly true if the specimen is not obtained from the same animal from which it was removed. Furthermore, if the cellular material cannot be removed by the body from within the basal lamina framework, it acts as a barrier to axon regeneration.
The need continues for improved grafts for connecting severed nerves, particularly for grafts that are generally useful for any recipient animal without serious potential of inducing an immune response.